1. Field of the Invention
The present invention relates to a coating composition for a coated electrode for arc welding, particularly a low-hydrogen coating composition for a coated electrode for arc welding, and a coated electrode for arc welding coated with the coating composition. The coated electrode of the present invention is particularly suitable for arc welding butt joint portions of a horizontal steel pipe with flat position, vertical-downward position and overhead position successively from the top to the bottom of the steel pipe.
2. Description of Prior Art
For welding butt joints of fixed horizontal steel pipes, a downward welding method which welds from the top to the bottom using a high-cellulose electrode and an upward welding method which welds from the bottom to the top using a low hydrogen electrode have been widely practised.
The high-cellulose electrode due to its advantages that the amount of slag produced is small and good penetration is obtained, so that a vertical downward welding is easily performed, with high welding efficiency, has been widely used for many years in Europe and America. However, in recent years as a high tensile strength steel pipe having a tensile strength not lower than 50 Kg/mm.sup.2 has been increasingly used, the high-cellulose electrode is often confronted with problems of root cracking and under-bead cracking due to hydrogen in the deposited metal.
These cracking may be prevented by preheating, but the preheating is troublesome and is not economical because it causes additional fuel cost.
Moreover, welded steel pipes are increasingly used in cold regions and in this application the high-cellulose electrode cannot give satisfactory notch toughness.
Meanwhile, the low-hydrogen electrode gives excellent notch toughness to the deposited metal, but as the upward welding method using this electrode is done with a current density 20 to 30% lower than that used in the downward welding method, the welding efficiency is lower about 50% than that of the downward welding method.
The conventional welding electrodes, as mentioned above, have their advantages and defects and have been unsuccessful in satisfying all requirements for welding a horizontal steel pipe.
Therefore, there has been strong demand for the development of an electrode which can give excellent notch toughness to the deposited metal and easily perform a downward welding with flat, vertical and overhead positions from the reverse-side-bead of the root pass to the cap-pass.